1. Field of the Invention
This invention relates to new magnesium and aluminum alloy articles consisting of a non-equilibrium matrix phase of essentially early, i.e. light rare earth and/or transition metals and/or metalloids made by non-equilibrium methods such as rapid solidification from the melt and from the vapor phase and by solid state synthesis with an essentially homogeneous distribution of the major part of the alloying elements on an atomic length scale of the eventually purified alloy matrix. More particularly, it relates to economically viable wrought magnesium and aluminum alloy articles made by selected processing routes and useful as extruded, forged or rolled products for space, ballistic, airframe and other aeronautical as well as for terrestrial applications such as in trains and automobiles, the products thereby achieved by novel methods to control the alloy synthesis, alloy conversion and alloy joining.
2. Description of the Related Art
Corrosion resistant commercial magnesium alloy such as the new high purity version of the Mg—Al base AZ91 alloy, i.e. AZ91E (8.3-9.7 Al, 0.35-1.0 Zn, <0.15 Mn, <0.1 Si, balance Mg) or the new Mg—Y base WE43-alloy (3.7-4.3 Y, 2.4-4.4 Nd and heavy rare earth misch-metal, 0.4-1.0 Zr, <0.2 Zn, balance Mg) are comparable with the corrosion rates of pure magnesium, of aluminum alloys A357 and A206 (all with corrosion rates of the order of 0.25-0.51 mm/year (10-20 mils per year [mpy]) in a salt fog test after ASTM B117) and they are about two orders of magnitude better then previous magnesium alloy families (cf. J. F. King, New advanced magnesium alloys, Advanced materials technology int., 1990, pp. 12-19). Another new magnesium alloy showing about 0.25 mm/year (10 mpy) in standardized test conditions is the rapidly solidified magnesium alloy EA55RS (5.1 Al, 4.9 Zn, 5.0 Nd, balance Mg) which has been made available quite recently as a wrought alloy product in extruded, rolled and forged form and which allows due to the fine grain structure for superplasticity and an alloy forming operation at about 150° C. lower temperatures than conventionally cast magnesium alloys so retaining the refined microstructure and the resultant improvement of engineering properties in the final product (S. K. Das, C. F. Chang and D. Raybould, PM in Aerospace and Defense Technologies, edt. F. H. Froes, MPIF, Princeton, N.J. 08540, 1989, pp. 63-66). On the aluminum side, many new alloy compositions with superior properties have been developed, but the methods to synthesize them from the vapor and solid state are not mature and controllable as is required by (pilot) production scale.
Aerospace applications require metallic materials with self-healing surface films to protect the interior, i.e. the bulk material when exposed to air (including rain independent on environmental particulars). None of the existing magnesium engineering alloys exhibit a surface passivation upon exposure to normal atmospheres containing saline species as it is known for titanium and aluminum alloys. For iron it is the allotropy which allows for passivation by equilibrium alloying austenitic and ferritic iron with chromium, for example. The absence of allotropy for aluminum, for example, results in deterioration of corrosion behavior of aluminum upon equilibrium alloying and this applies more seriously to magnesium alloys. Magnesium alloys yet represent the worst case among structural metals for aeronautical applications, since magnesium has not only no allotropy as titanium and iron, but Magnesium does also not develop a passive surface film on exposure to normal atmospheres as is evident for pure titanium and pure aluminum. None of the existing conventional magnesium alloys have yet shown pronounced passivation behavior by alloying as—by definition—becomes evident upon a significant decrease in corrosion rates compared to the pure metal. Hehmann et al. have shown (F. Hehmann, R. G. J. Edyvean, H. Jones and F.
Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1), however, that significant passivation is possible by alloying the αMg solid solution with at least 17 wt. % Al in the supersaturated state. This type of passivation, however, was not obtainable unless very extreme conditions of rapid solidification from the melt were applied and it was therefore restricted to thin cross-sections and not obtainable by conventional ingot metallurgy. An engineering solution to this problem would provide the driving force to resolve many of the obstacles for the introduction of advanced light alloys, but the solution to this problem has not been recognized as a combined problem of the development of non-equilibrium new and/or established light alloys as well as of corresponding processes.
As long as 75 years ago, Tammann (G. Tammann, Die chemischen und galvanischen Eigenschaften von Mischkristallen und ihre Atomverteilung, Leipzig, 1919) and later Gerischer et al., (cf. R. P. Tischer and H. Gerischer, Z. Electrochem. 62, 1958, p. 50.) reported increasing pitting potentials and decreasing anodic current densities of the equilibrium Cu—Au and Ag—Au solid solutions with increasing levels of gold representing the more noble and passivating constituent. The majority of the equilibrium phase diagrams of binary Mg-alloys shows, however, a very restricted solubility range in the cph-Mg solid solution due to the formation of strong compounds suppressing equilibrium solubility in cph-Mg (L. A. Carapella, Fundamental Alloying Nature of Magnesium, Met. Progress 48, August 1947, pp. 297-307). Only the so-called “yttrics” exhibit relatively large equilibrium solid solubilities in cph-Mg. This group consists of yttrium and the heavy rare earth metals Gd, Tb, Dy, etc. as well as scandium which, due to their physical commonalties, are found in nature as a mixture, the so-called (heavy) rare earth (HRE) misch-metals and which have led to the most heat resistant Mg-based alloys on record. Heavy rare earth metals and scandium are relatively expensive alloying additions to magnesium. Sm and Gd represent the most economically viable individual heavy rare earth alloying additions with relatively large equilibrium solid solubility in cph-Mg. If Sm and Gd are employed via a cheaper misch-metal, they may co-exist with a considerable amount of yttrium.
Yttirium, however, was reported (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, Rasch Erstarrte Magnesium-Mischkristalle und Ihr Umwandlungs-und Korrosionsverhalten, Doctoral Thesis, University of Stuttgart, published in Fortschrittsberichte VDI', Reihe 5, No 155: Grund-und Werkstoffe', VDI-Verlag, Düsseldorf, F. R. G., January 1989) not to result in the required improvement of corrosion behavior when dissolved in cph-Mg compared to pure magnesium. Mg-HRE alloys require also relatively laborious solution and aging treatments when made by conventional casting methods (cf. M. E. Drits, L. L. Rohklin and N. P. Abrukina, Metallovedenie i Termicheskaya Obrabotka Metallov 17, 1985, 27-28; S. Kamado, Y. Kojima, Y. Negishi and S. Iwasawa, R. Ninomiya, Light Metals Processing and Applications, Quebec City, Quebec Canada, 29 Aug.-1 Sep. 1993, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Quebec H3Z 3B8, Canada, 1993, pp. 849-858).
In 1987, Hehmann and co-workers found (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, Rasch Erstarrte Magnesium-Mischkristalle und Ihr Umwandlungs-und Korrosionsverhalten, Doctoral Thesis, University of Stuttgart, published in Fortschrittsberichte VDI', Reihe 5, No 155: Grund-und Werkstoffe', VDI-Verlag, Düsseldorf, F. R. G., January 1989) that the rapidly solidified (RS) solid solutions of La and Ce in the as-quenched supersaturated cph-Mg were very effective in order to passivate magnesium and to reduce corrosion rates compared to the pure metal. Only very small levels of La and Ce, i.e. 0.4 at. % La or Ce (2.2 wt. % La or Ce) in cph-Mg were required to arrive at uniform corrosion rates as low as 0.04 mils/yr (1 μm/year) compared to 15-20 mils/yr (350-500 μm/year) for pure magnesium by using a 1 mmol aerated NaCl aqueous solution of PH=4.9 (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, Rasch Erstarrte Magnesium-Mischkristalle und Ihr Umwandlungs-und Korrosionsverhalten, Doctoral Thesis, University of Stuttgart, published in Fortschrittsberichte VDI', Reihe 5, No 155: Grund-und Werkstoffe', VDI-Verlag, Düsseldorf, F. R. G., January 1989). Similar effects were observed before during the polarization of 7075 aluminum alloy in an electrolyte that was doped with La- and Ce-salts (B. R. W. Hinton, N. E. Ryan, D. R. Arnott, P. N. Trathen, L. Wilson and B. E. Williams, Corrosion Australasia 10, vol. 3, 1985, pp. 12-17). By contrast, the passivation of an cph-Mg-base solid solution containing Al required an Al-level of more than 16 wt. % in the supersaturated cph-Mg-state in order to arrive at 7 mils/yr (200 μm/yr) (F. Hehmann, H. Jones, F. Sommer and R. G. J. Edyvean, Corrosion Inhibition in Magnesium-Aluminium Based Alloys Induced by Rapid Solidification Processing, J. Mater. Sci. 24, 1989, pp. 2369-2379) and corresponding solid solutions were thermally very unstable (F. Hehmann, Metastable Phase Transformation in Rapidly Solidified Magnesium-Base Mg—Al Alloys, Acta Met. Mater. 38, 1990, pp. 979-992). Light RE-metals are not only cheaper than heavy RE-metals. They also showed a better performance compared to the passivation effects obtainable by heavy RE elements in cph-Mg via conventional casting methods (cf. F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1. vs. L. A. Carapella, Fundamental Alloying Nature of Magnesium, Met. Progress 48, August 1947, pp. 297-307; S. Kamado, Y. Kojima, Y. Negishi and S. Iwasawa, R. Ninomiya, Light Metals Processing and Applications, Quebec City, Quebec Canada, 29 Aug.-1 Sep. 1993, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Quebec H3Z 3B8, Canada, 1993, pp. 849-858). Alloying of the cph-Mg based solid solution with light RE elements seems to provide a very effective alternative to passivate magnesium metal used as a matrix material.
For passivation of magnesium, the details of the microstructure appeared to be crucial in addition to the solute selected for solid solution alloying with cph-Mg and to the concentration range of corresponding solid solution. The reduction in corrosion current by several orders of magnitude compared to corresponding ingot castings and prior-art Mg engineering alloys was considered (cf. F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, ‘Rasch Erstarrte Magnesium-Mischkristalle und Ihr Umwandlungs-und Korrosionsverhalten’, Doctoral Thesis, University of Stuttgart, published in Fortschrittberichte VDI', Reihe 5, No 155: Grund-und Werkstoffe', VDI-Verlag, Düsseldorf, F. R. G., January 1989) to originate in the complete absence of second phases, i.e. when the volume fraction of equilibrium dispersoids separated from the melt upon solidification was virtually 0.0 and the RE elements virtually completely held in the cph-Mg solid solution. However, relatively early pitting corrosion was also observed despite significant reduction in the more uniform corrosion rate. A precise correlation between the various non-equilibrium microstructures of these alloys and their response to corrosive attack has yet not been forwarded. Moreover, the corrosion behavior of theses microstructures upon exposure to test conditions which are accepted by industry has also not yet been presented.
A similar behavior was observed with Al—Cr—Fe alloys made by vapor deposition. These alloys do not know any larger volume fraction of second phases as would apply to the equilibrium state of corresponding compositions (M. C. McConnell and P. G. Partridge, Processing of Structural Metals by Rapid Solidification, eds. F. H. Froes and S. J. Savage, American Society for Metals, Metals Park, Ohio, 1987, pp. 143-153; R. L. Bickerdike, D. Clark, G. Hughes, M. C. McConnell, W. N. Mair, P. G. Partridge and B. W. Viney, Int. Conf. Rapidly Solidified Materials, San Diego, ASM Metals Park, 1986, pp. 145-151; P. G. Partridge, Processing of Structural Metals by Rapid Solidification, eds. F. H. Froes and S. J. Savage, American Society for Metals, Metals Park, Ohio, 1987, pp. 155-162) While (all) Al-alloys show deterioration of the corrosion resistance relative to (commercially and/or high and/or ultra-) pure aluminum due to the microgalvanic effect(s) at the alloy surface, the PVD-Al—Cr—Fe alloys showed threefold improved corrosion resistance over pure aluminum.
The extension of equilibrium solid solubility of light rare earth elements in cph-Mg require high front velocities to suppress microsegregations upon solidification of the melt due to low partition coefficients ko(T). ko(T) is defined as the ratio CS/CL at a given temperature T, where Cs=solidus concentration and CL=liquidus concentration of an initial alloy concentration co. Corresponding values range from 0.05 for Mg-Eu to 0.1 for Mg—Sm (cf. F. Hehmann, F. Sommer and H. Jones, Extension of Solid Solubility of Yttrium and Rare Earth Metals in Magnesium by Rapid Solidification, Processing of Structural Metals by Rapid Solidification, eds. F. H. Froes and S. J. Savage, American Society for Metals, Metals Park, Ohio, 1987, pp. 379-398; F. Hehmann, F. Sommer and B. Predel, Extension of Solid Solubility in Magnesium by Rapid Solidification, Mat. Sci. Engng. A125 (2), 1990, pp. 249-265). Hypoeutectic Mg—Sr alloys with coefficients ko of 0.005 were shown to require front velocities of 2 to 4 m/s corresponding to laser withdrawal velocities of 3 to 6 m/s in order to achieve solidification without microsegregations (F. Hehmann and P. Tsakiropoulos, Microstructural Modelling of Lazer Glazing, Gas-Atomization and Spray Forming for the Development of Magnesium Alloys, Conf. Proc. Magnesium Alloys and Their Applications, DGM, Oberursel, FRG, 1992). The conditions to extend the equilibrium solid solubility of light RE metals in cph-Mg by liquid quenching methods are therefore not readily available.
Corresponding one phase as-solidified microstructure was observed in a surface chill zone of width 20-30 μm of piston-and-anvil (PA) splats of overall thickness 150 μm (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, Rasch Erstarrte Magnesium-Mischkristalle und Ihr Umwandlungs-und Korrosionsverhalten, Doctoral Thesis, University of Stuttgart, published in Fortschrittsberichte VDI', Reihe 5, No 155: Grund-und Werkstoffe', VDI-Verlag, Düsseldorf, F. R. G., January 1989). The reminding cross-section of dendritic growth of equilibrium phases was a result of the recalescence triggered by internal release of latent heat that occurs when the solidification front traverses the cross-section of the volume flattened by “splatting”. PA-splat cooling is a discontinuous method to produce small volumes of material. In order to achieve a surface chill zone of width 20-30 μm in a sample of size 50 mg, pressures up to 5 bar for pneumatic acceleration of the piston were required (cf. H. Gronert, Dipl. Thesis, University of Duisburg, 1984). Due to the variety of microstructures accrued to the high pressure available, PA-splat cooling is a very useful method to evaluate the departure from microstructural and structural equilibrium required for the economically viable production of passive magnesium alloys by using continuous RS-manufacturing methods. However, the high quality of these microstructural portions were instrumental for the present invention.
One RS-processing method that could continuously produce metastable phases and microstructures is vapor deposition. Bray et al. reported (D. J. Bray, R. W. Gardiner, B. W. Viney and H. M. Flower, Conf. Proc. Magnesium Alloys and Their Applications, DGM, Oberursel, FRG, 1992, pp. 159-166; D. J. Bray, R. W. Gardiner and B. W. Viney, GB-Patent 2,262,539 A, 23 Jun. 1993) on extension of prior-art by the effect of titanium in extended solid solution of cph-Mg made by thermal evaporation on to a collector which was temperature-controlled at between 100°-150° C. The Mg—Ti system was identified (D. J. Bray, R. W. Gardiner and B. W. Viney, GB-Patent 2,262,539 A, 23 Jun. 1993) to develop annual corrosion rates between 330 μm/yr for Mg-2.0 wt. % Ti over 30 μm/yr for Mg-22 wt. % Ti and 5 μm/yr for Mg-47 wt. % Ti compared to 490 μm/yr for evaporated pure magnesium and 420 μm/yr for WE43, for example, as was derived from weight loss experiments after immersion for 7 days in 0.6 mol NaCl aqueous solution. The disadvantages of vapor deposition of Mg—Ti base alloys appeared to include 1. a thermally relatively unstable solid solution of at least a substantial part of Ti in cph-Mg, i.e. not much higher than 200° C., 2. that significant passivation required Ti-levels as high as 22 wt. % where the density of the overall alloy had already exceeded a value of 2.0 g/cm3 and 3. that Ti is very different from magnesium in that it provides a much higher vapor pressure so evaporating not as easily to provide an economically viable major alloying addition to cph-Mg. Ti is a representative for early transition metals to produce new and corrosion resistant magnesium base alloys showing the importance to develop relevant vapor deposition processes.
The non-equilibrium microstructures offered by the umbrella of rapid solidification processing (RSP) have yet not been explored systematically in order to develop magnesium alloys and applications with significantly improved surface passivity. The alloy compositions and the possibilities for conversion of non-equilibrium Mg-alloys into products of which corrosion resistance, mechanical properties, the stability and the transformation behavior of the metastable non-equilibrium state are of prime concern, have also not been explored to date. This concerns wrought products with a fine grain size inside the bulk material and which are suitable for low and elevated temperature applications as well as for weathered applications.